Cathodized gold nanoparticle graphite pencil electrode and method for glucose detection

ABSTRACT

The cathodized gold nanoparticle graphite pencil electrode is a sensitive enzymeless electrochemical glucose sensor based on the cathodization of AuNP-GPE. Cyclic voltammetry shows that advantageously, the cathodized AuNP-GPE is able to oxidize glucose partially at low potential (around −0.27 V). Fructose and sucrose cannot be oxidized at &lt;0.1 V, thus the glucose oxidation peak at around −0.27 V is suitable enough for selective detection of glucose in the presence of fructose and sucrose. However, the glucose oxidation peak current at around −0.27 V is much lower which should be enhanced to obtain low detection limit. The AuNP-GPE cathodization increases the oxidation peak current of glucose at around −0.27 V. The dynamic range of the sensor is in the range between 0.05 to 5.0 mM of glucose with good linearity (R 2 =0.999). Almost no interference effect was observed for sensing of glucose in the presence of fructose, sucrose and NaCl.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to glucose sensors, and particularly to anenzyme-free cathodized gold nanoparticle graphite pencil electrode (GPE)based glucose sensor and methods for glucose detection.

2. Description of the Related Art

Glucose is an important molecule for human, plant and other livingorganisms. However, the presence of lower or higher concentration ofdissolved glucose in blood outside of the normal range (4.4-6.6 mM) isthe symptom of diseases “Diabetes mellitus”. As a result, knowing theexact glucose level in blood is crucial for diagnosis and management ofDiabetes mellitus. Moreover, glucose is used in several industries suchas textile, pharmaceuticals, food industries including beverages,renewable and sustainable fuel cells, and the like. Therefore, a simple,disposable, cheap, selective and sensitive glucose sensor is requiredfor continuous glucose monitoring.

Among the common analytical methods, the electrochemical method has beenwidely appreciated due to its simplicity, portability, selectivity andsensitivity. Generally, electrochemical glucose sensors are classifiedas either (i) enzyme base glucose sensor or (ii) nonenzymatic glucosesensor. Expensive enzyme and complicated enzyme immobilization methodsare required for fabrication of enzyme based electrochemical glucosesensors. Moreover, H₂O₂ is produced in the enzyme base glucose sensorfrom glucose and the produced H₂O₂ is oxidized on the electrode surfaceto generate a signal for the glucose. Practically, for oxidation of H₂O₂there is typically required a potential which is high enough to oxidizeinterference (e.g. fructose, sucrose, ascorbic acid, dopamine uric acidetc.) in the real sample.

To overcome those problems, a plethora of nonenzymatic glucose sensorshave been developed. A nonenzymatic glucose sensor depends on a directglucose oxidation signal on the electrode surface and their selectivitydepends on the oxidation potential of glucose. Nanoparticles of bothtransition and noble metals have been used to enhance theelectrocatalytic properties of a substrate electrode toward glucoseoxidation. For example, gold nanowire array electrode, goldnanoparticles (Au NPs)-modified amine-functioned mesoporous silica filmson glassy carbon electrode (GCE), CoOOH nanosheet-modified cobaltelectrode, bimetallic Pt—M (M=Ru and Sn) NPs on carbon nanotube(CNT)-modified GCE, Pt/Ni—Co nanowires, Pd NPs on graphene oxide, Au NPson polypyrrole nanofibers-modified GCE, copper NPs on CNT-modified GCE,Au NP-modified nitrogen-doped diamond-like carbon electrodes,AuNP/carbon nanotubes/ionic liquid nanocomposite, and Au NP-modifiedindium tin oxide were used to direct oxidation of glucose.

Glucose can be partially oxidized at a bulk Au electrode or a nano goldelectrode at lower potential which is required to eliminate theinterferences effect for detecting glucose in a real sample. However,the signal of partial oxidation of glucose in alkaline medium at Auelectrode is lower than that of full oxidation at high potential. It isknown that the high signal is required to obtain low detection limit inan electrochemical sensor, and that the cathodization of an Aunanomaterial based electrode before recording an electrochemical signalcan enhance the electrochemical signal of the analyte. The reasons oflimited use of Au electrode for routine analysis of glucose are highprice of gold, complex preparation method of nano gold or nanogold-modified electrode and low signal at low potential.

Moreover, the graphite pencil electrode (GPE) is an attractive electrodematerial because it is cheap, available, possesses an easy to makerenewable surface, and is relatively stable. However, graphite typicallyshows poor electrocatalytic properties toward many electroactivemolecules. The poor electrocatalytic properties of GPE should beimproved to obtain a lower detection limit in electrochemical sensors.

Thus, a cathodized gold nanoparticle graphite pencil electrodeaddressing the aforementioned problems is desired.

SUMMARY OF THE INVENTION

Embodiments of a cathodized gold nanoparticle graphite pencil electrode(AuNP-GPE) provide a highly sensitive enzymeless electrochemical glucosesensor that is based on the cathodized gold nanoparticle-modifiedgraphite pencil electrode (AuNP-GPE). By combining the advantages ofAuNP, GPE and cathodization, embodiments of an AuNP-GPE provide for thefabrication of a nonenzymatic highly selective and relatively sensitive,cheap and disposable glucose sensor. The AuNP-GPE after cathodization atan optimum condition shows relatively a high selectivity, a lowdetection limit (12 micromolar (μM)) and a wide dynamic range (0.05-5millimolar (mM)) toward glucose sensing. The cyclic voltammetry (CV)experiments show that embodiments of an AuNP-GPE can oxidize glucosepartially at low potential (around −0.27 volts (V)), whereas the bareGPE generally cannot oxidize glucose in the entire tested potentialwindows, and that fructose and sucrose generally cannot be oxidized at<0.1 V at an AuNP-GPE. As a result, the glucose oxidation peak at around−0.27 V is relatively suitable enough for selective detection of glucosein the presence of fructose and sucrose.

However, the glucose oxidation peak current at around −0.27 V istypically much lower which should be enhanced to obtain a low detectionlimit. To increase the oxidation peak current of glucose at around −0.27V, embodiments of an AuNP-GPE have been cathodized under relativelyoptimum condition (−1.0 V for 30 seconds (s)) in the same glucosesolution before recording cyclic voltammetry (CV). This cathodization ofan AuNP-GPE enhances the glucose signal and can allow a detection limitof 12 μM of glucose, for example. The dynamic range of embodiments of aglucose sensor using embodiments of a cathodized AuNP-GPE are typicallyin the range between 0.05 to 5.0 mM of glucose with relatively goodlinearity (R²=0.999). Also, no significant interference effect wasobserved for the sensing of glucose in the presence of fructose, sucroseand NaCl.

These and other features of the present invention will become readilyapparent upon further review of the following specification anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a Field Emission Scanning Electron Microscope (FE-SEM) imageat 2 μm for a known bare graphite pencil electrode (GPE).

FIG. 1B is a FE-SEM image at 2 μm for an AuNP-GPE electrode according tothe present invention.

FIG. 1C is a FE-SEM image at 200 nm for a known bare GPE.

FIG. 1D is a FE-SEM image at 200 nm for an AuNP-GPE according to thepresent invention.

FIG. 2A is a CV plot of a bare GPE versus an AuNP-GPE at a scan rate of100 millivolts/second (mV/s) in the absence of glucose according to thepresent invention.

FIG. 2B is a CV plot of a bare GPE versus an AuNP-GPE at a scan rate of100 mV/s in the presence of glucose according to the present invention.

FIG. 2C is a CV plot of a bare GPE versus an AuNP-GPE at a scan rate of100 mV/s in the presence of fructose according to the present invention.

FIG. 2D is a CV plot of a bare GPE versus an AuNP-GPE at a scan rate of100 mV/s in the presence of sucrose according to the present invention.

FIG. 3A is a plot of CVs in the absence of glucose at a bare GPE, beforeversus after cathodization. Scan rate: 100 mV/s.

FIG. 3B is a plot of CVs in the presence of glucose at a bare GPE beforeversus after cathodization. Scan rate: 100 mV/s.

FIG. 3C is a plot of CVs in the absence of glucose at an AuNP-GPE,before versus after cathodization, at a scan rate of 100 mV/s.

FIG. 3D is a plot of CVs in the presence of glucose at an AuNP-GPE,before versus after cathodization. Scan rate: 100 mV/s.

FIG. 4A is a plot of anodic sweeps of CVs in the presence of glucose atan AuNP-GPE after cathodization at different potentials. Scan rate: 100mV/s.

FIG. 4B is a plot of peak current versus cathodization potential of FIG.4A.

FIG. 5A is a plot of anodic sweeps of CVs in the presence of glucose atan AuNP-GPE after cathodization at different times.

FIG. 5B is a plot of peak current versus cathodization time of FIG. 5A.

FIG. 6A is a plot of anodic sweeps of CVs at different scan rates in thepresence of glucose at a cathodized AuNP-GPE.

FIG. 6B is a plot of peak current versus scan rate of FIG. 6A.

FIG. 7A is a plot of anodic sweeps of CVs in the presence of variousconcentrations of glucose at a cathodized AuNP-GPE.

FIG. 7B is the corresponding calibration curve of FIG. 7A.

FIG. 8A is a plot of anodic sweeps of CVs of fructose in the absence andpresence of glucose at a cathodized AuNP-GPE.

FIG. 8B is a plot of anodic sweeps of CVs of sucrose in the absence andpresence of glucose at a cathodized AuNP-GPE.

FIG. 8C is a plot of anodic sweeps of CVs of sodium chloride in theabsence and presence of glucose at a cathodized AuNP-GPE.

Unless otherwise indicated, similar reference characters denotecorresponding features consistently throughout the attached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of a cathodized gold nanoparticle graphite pencil electrode(AuNP-GPE) 10 b (shown in FIGS. 1B and 1D in the micrographs 100 b and100 d, respectively) provides a relatively highly sensitive enzymelesselectrochemical glucose sensor based on a cathodized goldnanoparticle-modified graphite pencil electrode (AuNP-GPE). Performingthe cyclic voltammetry (CV) experiments show that embodiments of anAuNP-GPE can oxidize glucose partially at a relatively low potential(around −0.27 V), whereas the bare GPE 10 a (shown in FIGS. 1A and 1C inthe micrographs 100 a and 100 c, respectively) typically cannot oxidizeglucose in the entire tested potential windows. Besides, fructose andsucrose generally cannot be oxidized at <0.1 V at an AuNP-GPE. As aresult, the glucose oxidation peak at around −0.27 V is generallysuitable enough for selective detection of glucose in the presence offructose and sucrose, for example.

However, the glucose oxidation peak current at around −0.27 V is muchlower which should be enhanced to obtain a low detection limit. Toincrease the oxidation peak current of glucose at around −0.27 V, theembodiments of an AuNP-GPE have been cathodized under a relativelyoptimum condition (−1.0 V for 30 s) in the same glucose solution beforeperforming and recording cyclic voltammetry (CV). This cathodizationenhances the glucose signal and allows for a glucose detection limit of12 μM, for example. The dynamic range of the sensor is in the rangebetween 0.05 to 5.0 mM of glucose with a relatively good linearity(R²=0.999). Also, no significant interference effect was observed forsensing of glucose in the presence of fructose, sucrose and NaCl, forexample.

With respect to reagents used in relation to preparation of embodimentsof an AuNP-GPE, Gold(III) chloride hydrate, D-(+) Glucose, D-(−)Fructose, Sucrose, L-ascorbic acid (AA), Sodium chloride and Sodiumhydroxide were received from Sigma Aldrich. As to an example of graphiteused in relation to preparation of embodiments of an AuNP-GPE,hi-polymer graphite graphite pencil HB (grade) black leads were obtainedfrom Pentel Co. LTD. (Japan). All leads had a total length of 60millimeters (mm) and a diameter of 0.5 mm, and were used as received.All solutions were prepared with deionized water of a resistivity of18.6 megaohms/centimeter (MΩ/cm), which was obtained directly fromPURELAB® Ultra Laboratory Water Purification System.

A Jedo mechanical pencil (Korea) was used as a holder for both bare andAuNP-modified graphite pencil leads. Electrical contact with the leadwas achieved by soldering a copper wire to the metallic part that holdsthe lead in place inside the pencil to provide an electricallyconductive holder. The pencil lead was fixed vertically with 15 mm ofthe pencil lead extruded outside, and 10 mm of the lead immersed in thesolution. Such length corresponds to a geometric electrode area of 15.90mm². CH Instruments Inc. instrumentation was used for theelectrochemical work in relation to embodiments of an AuNP-GPE. Theelectrochemical cell contained a bare GPE or an AuNP-GPE as a workingelectrode, a Pt wire counter electrode and Ag/AgCl (Sat. KCl) referenceelectrode. Before recording each voltammogram, argon gas was bubbled for30 minutes (min) to remove oxygen from the solution. The FE-SEM imageswere recorded using TESCAN LYRA 3 at Center of Research Excellence inNanotechnology (CENT), King Fahd University of Petroleum and Minerals(KFUPM), Kingdom of Saudi Arabia.

With respect to embodiments of preparation methods of an AuNP-GPE,briefly, initially equal volumes (1.5 milliliters (ml) of each aqueoussolutions) of 1.65 mM AA and 1.0 mM Gold(III) chloride were mixed usinga pipette at room temperature (RT) in a 3.0 ml test tube to form goldnanoparticles (AuNPs). A bare GPE was immersed into that test tube,which was placed into a water bath preheated to 75° centigrade (C) andkept for 15 min to obtain the AuNP-GPE. Afterward, the AuNP-GPE wasremoved and washed by gentle dipping two times in deionized water, thendried at 60° C. for 5 min prior to use. The prepared AuNP-GPE was thencathodized by placing the AuNP-GPE in a glucose analyte solution andapplying −1.0 volt to the AuNP-GPE for approximately 30 seconds toprovide a cathodized AuNP-GPE. Also, the prepared AuNP-GPE can also becathodized by placing the AuNP-GPE in a basic solution, such as in a 0.1molar (M) NaOH, at −1.0 V for 30 seconds.

FIGS. 1A and 1B illustrate 2 μm scanning electron microscope (SEM)images 100 a and 100 b of the bare GPE 10 a and the AuNP-GPE 10 b,respectively. Comparing between the SEM images of 100 a and 100 b, theeffect of the presence of AuNP is easily visible. The diameter of theAuNP is in the range of 20-85 nanometers (nm), for example. The lowmagnification view of the AuNP-GPE 10 b indicates that the AuNPs arerelatively evenly dispersed on the surface of the GPE. Higher resolutionimages 100 c and 100 d of the bare GPE 10 a vs. the AuNP-GPE 10 b at 200nm are shown in FIGS. 1C and 1D, respectively.

With respect to electrocatalytic oxidation of glucose, fructose andsucrose, in FIGS. 2A to 2D, CVs in a 0.1 M NaOH in the absence (FIG. 2A)and presence (FIG. 2B) of 1 mM D-(+) glucose, and in the presence ofD-(−) fructose (FIG. 2C), and in the presence of sucrose (FIG. 2D) at abare GPE (“a” plots in FIGS. 2A to 2D) and at an AuNP-GPE (“b” plots inFIGS. 2A to 2D), and at a scan rate of 100 mV/s are illustrated. Plot200 a of FIG. 2A and plot 200 b of FIG. 2B are the CVs in 0.1 M NaOH ata bare GPE and at an AuNP-GPE. In comparison to the CV of bare GPE, itis clear that AuNP-GPE started to oxidize around at −0.1 V in an anodicsweep and the oxidized Au is subsequently reduced at a cathodic sweepwith a reduction peak at around +0.12 V, for example. Interestingly,there is relatively no large difference in the background current of anAuNP-GPE and a bare GPE in anodic sweep at <−0.1 V. Therefore, thedetection of glucose at <−0.1 V is generally good to obtain a lowdetection limit, for example. By comparing the “a” line plot of plots200 a-200 d in FIGS. 2A through 2D, respectively, it is clear that abare GPE cannot typically oxidize glucose, fructose, and sucrose in thetest potential windows. Moreover, the “b” line plot of plot 200 b inFIG. 2B presents the CV of glucose at an AuNP-GPE. From the comparisonbetween the “b” line plot of FIGS. 2A and 2B, it is evident that twopeaks at an anodic sweep and one peak at a cathodic sweep appeared forglucose oxidation. This typical glucose electrooxidation behavior inalkaline solution is similar with that at a bulk gold electrode.

While the mechanism of the glucose oxidation is relatively complex, theresults of the mechanism in relation to embodiments of an AuNP-GPE,indicate a glucose oxidation peak appears in an anodic sweep at around−0.27 (E_(pa1)), and +0.27 V (E_(pa2))) or in a cathodic sweep around at+0.12V (E_(pc)). Besides, the oxidation peak of fructose (the “b” lineplot in FIG. 2C) or sucrose (the “b” line plot in FIG. 2D) at anAuNP-GPE is in an anodic sweep around at +0.29 V and is in a cathodicsweep around at +0.15V. From the above discussion, it can be determinedthat in embodiments of an AuNP-GPE, the glucose oxidation peak at −0.27V is relatively desirable to detect the glucose without any significantinterference from fructose and sucrose with a minimum background currentwhich is typically desired to get a low detection limit. However, theglucose oxidation peak current at E_(pa1) is relatively much lower thanthat obtained at E_(pa2) or E_(pc). Also, a high peak current at E_(pa1)is generally desired for obtaining a low detection limit with arelatively high selectivity.

Referring to FIGS. 3A to 3D, CVs in 0.1 M NaOH in the absence (FIG. 3Aand FIG. 3C) and presence (FIG. 3B and FIG. 3D) of a 1 mM D-(+) glucoseat a bare GPE (FIG. 3A and FIG. 3B) and at an AuNP-GPE (FIG. 3C and FIG.3D) at a scan rate of 100 mV/s are illustrated. The CVs in FIGS. 3A-3Dwere recorded before (plots “a”) and after (plots “b”) cathodization ofthe electrodes at −1.0 V for 60 s. Regarding glucose oxidation signalenhancement by cathodization of AuNP-GPE in relation to embodiments ofan AuNP-GPE, initially, the cathodization effect on background currentof a bare GPE, as shown in plot 300 a of FIG. 3A and an AuNP-GPE, asshown in plot 300 c of FIG. 3C was checked. By comparing the “a” plotline and the “b” plot line of plot 300 a in FIG. 3A, it is observed thatthere was no significant change in a background current before and aftercathodization of a bare GPE. Similarly, no significant change is foundin a background current of a cathodized (“b” plot line of plot 300 c inFIG. 3C) and uncathodized (“a” plot line of plot 300 c in FIG. 3C)AuNP-GPE.

As evident from the “b” CV plots of FIGS. 3A and 3C, the backgroundcurrent of a cathodized bare GPE and of a cathodized AuNP-GPE issimilar. Also, a cathodized and an uncathodized bare GPE typicallycannot oxidize glucose at negative potential, which is confirmed bycomparing the “b” plot lines of FIGS. 3A and 3B. Comparing FIGS. 3C and3D and the “b” plot line of FIG. 2B shows that glucose can be oxidizedon uncathodized or cathodized AuNP-GPE at around −0.27 V. However, thesignal of glucose electrooxidation has been enhanced significantly at anAuNP-GPE after cathodization (the “b” plot line of plot 300 d in FIG.3D) compared to that of before cathodization (the “b” plot line of FIG.2B and the “a” plot line of FIG. 3D), for example.

With respect to optimization of cathodization parameters for glucoseoxidation, to obtain a highest signal of glucose, the cathodizationpotential and time were optimized. Referring to FIG. 4A, anodic sweepsof CVs in a 0.1 M NaOH containing 1 mM D-(+) glucose at an AuNP-GPE,after cathodization for 60 s at different potentials of (a)-0.2 V, (b)−0.6 V, (c) −0.8 V, (d) −1.0 V and (e) −1.2 V, and a scan rate of 100mV/s are illustrated; and FIG. 4B illustrates the corresponding plot ofpeak current vs. cathodization potential. Plot 400 a of FIG. 4Aillustrates an anodic sweep of CVs of 1 mM glucose which were obtainedat an AuNP-GPE after cathodization at different potentials for 60seconds in the same glucose solution. Plot 400 b of FIG. 4B illustratesthe corresponding plot of peak current versus cathodization potential.This plot indicates that −1.0 V is a desirable cathodization potentialfor achieving a relatively highest signal of glucose, for example.

Referring to FIG. 5A, anodic sweeps of CVs in a 0.1 M NaOH containing 1mM D-(+) glucose at an AuNP-GPE after cathodization at −1.0 V fordifferent times of (a) 5 s, (b) 15 s, (c) 30 s, (d) 45 s and (e) 60 s,at a scan rate of 100 mV/s are illustrated; and FIG. 5B illustrates thecorresponding plot of peak current versus cathodization time. Plot 500 aof FIG. 5A indicates an anodic sweep of CVs of 1 mM glucose which wereobtained at AuNP-GPE after cathodization at −1.0 V for the differenttimes “a” through “e” in the same glucose solution. The plot of peakcurrent versus cathodization time (plot 500 b of FIG. 5B) indicates thatthe relatively highest signal was obtained at an AuNP-GPE aftercathodization at −1.0 V for 30 seconds. As a result, −1.0 V and 30seconds were selected for cathodization of an AuNP-GPE in furtherexperiments.

Referring to FIG. 6A, anodic sweeps of CVs in a 0.1 M NaOH containing 1mM D-(+) glucose at an AuNP-GPE after cathodization at −1.0 V for 30 sat scan rates of (a) 10 mV/s, (b) 50 mV/s, (c) 100 mV/s, (d) 150 mV/s,(e) 200 mV/s, (f) 250 mV/s, (g) 300 mV/s, (h) 350 mV/s and (i) 400 mV/sare illustrated; and FIG. 6B illustrates the corresponding plot of peakcurrent versus scan rate. Regarding the effect of scan rate on glucoseoxidation in relation to embodiments of an AuNP-GPE, the relationshipbetween peak current and scan rate can be described as to theelectrochemical mechanism. Therefore, anodic sweeps of CVs of 1 mMglucose at a cathodized AuNP-GPE were recorded at different scan ratesfrom 10-400 mV/s (plot 600 a of FIG. 6A, at the scan rates “a” through“i”). Plot 600 b of FIG. 6B shows the corresponding plot of peak currentvs. scan rate. This plot shows that peak current has been linearlyincreased with increasing the scan rate. The plot follows the linearequations I_(p) (μA)=0.13ν (V/s)+15.92; R²=0.988. This indicates thatthe electrode process was controlled by adsorption rather thandiffusion, for example.

The reproducibility of the embodiments of glucose sensing methods usingembodiments of a cathodized AuNP-GPE was verified by recording the CVsat a scan rate of 300 mV/s in a 0.1 M NaOH containing 1 mM glucose at aseries of modified AuNP-GPE electrode surfaces after pretreatment at−1.0 V for 30 seconds. The intraday experiments showed a peak current of49.128±4.7190 μA (mean±standard deviation) with a relative standarddeviation of 9.6%, whereas the interday experiments showed a peakcurrent of 49.357±4.652 μA with a relative standard deviation of 9.43%.The results indicate that embodiments of a glucose sensing method usingembodiments of a cathodized AuNP-GPE are reproducible.

Regarding the effect of presence or absence of glucose duringcathodization, an AuNP-GPE was cathodized in a 0.1 M NaOH containing 1mM glucose solution, followed by recording the CV of glucose oxidationin the same solution. However, to compare the effect of glucose towardthe glucose oxidation reaction if the electrode is cathodized in theabsence of an analyte, the AuNP-GPE was also cathodized only in 0.1 MNaOH at −1.0 V for 30 seconds. Afterward, 1 mM equivalent amount ofglucose solution was added to the 0.1 M NaOH. A comparison of theresults indicates that a substantially same level of the glucoseoxidation signal was obtained for both cases. These phenomena indicatethat cathodization of an AuNP-GPE changes the electrocatlytic propertiesfor the enhancing of the glucose oxidation signal, rather than theaccumulation of glucose during the negative potential treatment.

Referring to FIG. 7A, anodic sweeps of CVs in a 0.1 M NaOH containingdifferent mM concentrations of D-(+) glucose at an AuNP-GPE aftercathodization at −1.0 V for 30 s, at (a) 0.0 mM, (b) 0.05 mM, (c) 0.1mM, (d) 0.25 mM, (e) 0.50 mM, (f) 0.75 mM, (g) 1.0 mM, (h) 2.0 mM and(i) 3.0 mM D-(+) glucose at a scan rate of 300 mV/s are illustrated; andFIG. 7B illustrates the corresponding calibration curve. With respect tovoltammetric determination of glucose the glucoseconcentration-dependent CVs (plot 700 a of FIG. 7A at the concentrations“a” through “i”) were recorded at an AuNP-GPE after cathodization in therespective glucose solution at −1.0 V for 30 seconds to determine thelimit of detection.

The concentration dependence calibration curve (plot 700 b of FIG. 7B)was constructed from the signal after subtracting the mean of the zeroglucose response. The calibration plot shows that the glucose oxidationsignals increase linearly with increasing concentration of glucose in arange between 0.05 mM to 5 mM. The calibration plot follows the linearregression equations, I_(p)=52.613 [glucose]+0.2066; R²=0.999. However,a further increase of the concentration to more than 5 mM results in thesignal showing a non linear behavior with concentration. The calculatedlimit of detection at 3σ was 12 μM glucose, for example. This limit ofdetection is comparable for a glucose sensor based on the Aunanomaterial-modified carbon electrode, for example.

Fructose, sucrose and NaCl coexist with glucose in many samplesincluding food and drugs. The fructose, glucose and NaCl can potentiallyinterfere with the glucose oxidation signal. Therefore, the effects ofthe presence of a 1 mM fructose or sucrose or NaCl on oxidation of 1 mMglucose at embodiments of an AuNP-GPE after cathodization in therespective glucose solution at −1.0 V for 30 seconds were studied.Referring to FIGS. 8A to 8C, anodic sweeps of CVs in a 0.1 M NaOHcontaining 0.1 mM D-(−) fructose (FIG. 8A) in the absence (plot “a”) andpresence (plot “b”) of a 1 mM D-(+) glucose, a 0.1 mM sucrose (FIG. 8B)in the absence (plot “a”) and presence (plot “b”) of a 1 mM D-(+)glucose, a 1 mM NaCl (FIG. 8C) in the absence (plot “a”) and presence(plot “b”) of a 1 mM D-(+) glucose at an AuNP-GPE after cathodization at−1.0 V for 30 s and at a scan rate of 300 mV/s are illustrated.

The “a” plot line shown in plots 800 a, 800 b, and 800 c of FIGS. 8A, 8Band 8C, respectively, depicts the anodic sweeps of CVs of 1 mM fructose,sucrose and NaCl, respectively. The CV data shows that fructose, sucroseand NaCl typically cannot generate any signal in the tested potentialwindows, whereas a 1 mM glucose in the absence (plot “g” of FIG. 7A) orpresence of 1 mM fructose (plot “b” of FIG. 8A) or sucrose (plot “b” ofFIG. 8B) or NaCl (plot “b” of FIG. 8C) can generate similar glucoseoxidation signals. The results indicate that embodiments of a methodusing embodiments of an AuNP-GPE are valid for the detection of glucosein the presence of fructose, sucrose and NaCl without any substantialinference.

Embodiments of a cathodized AuNP-GPE provide a sensitive, selective,relatively inexpensive and disposable glucose sensor based on acathodized AuNP-GPE. The cathodized AuNP-GPE shows relatively superiorelectrocatalytic properties toward electroxidation of glucose comparedto an uncathodized AuNP-GPE or a bare GPE. The selectivity of theglucose sensor was obtained by selecting the appropriate potentialwindows of a CV. A limit of detection of the embodiments of the AuNP-GPEsensor is 12 μM of glucose, for example. For a significantly lowdetection limit, greater analytical selectivity and sensitivity andrelatively low cost, embodiments of a method using embodiments of acathodized AuNP-GPE based on cathodization of a relatively simplyprepared AuNP-GPE can be suitable for analytical determination ofglucose in various fields.

It is to be understood that the present invention is not limited to theembodiments described above, but encompasses any and all embodimentswithin the scope of the following claims.

We claim:
 1. A cathodized gold nanoparticle graphite pencil electrode,comprising: a graphite electrode; and gold nanoparticles chemicallydisposed on a said graphite electrode to form a gold nanoparticlemodified graphite pencil electrode (AuNP-GPE), said AuNP-GPE beingsubjected to a negative voltage to cathodize said AuNP-GPE before use ofsaid AuNP-GPE as a voltammetric sensor in a glucose analyte solution. 2.The cathodized gold nanoparticle graphite pencil electrode according toclaim 1, wherein said AuNP-GPE is sensitive to the oxidation of glucosearound approximately −0.27 volts.
 3. The cathodized gold nanoparticlegraphite pencil electrode according to claim 1, wherein said AuNP-GPE iscathodized by applying −1.0 volts to said AuNP-GPE for approximately 30seconds.
 4. The cathodized gold nanoparticle graphite pencil electrodeaccording to claim 3, wherein said AuNP-GPE is sensitive to theoxidation of glucose around approximately −0.27 volts.
 5. A method forforming a cathodized gold nanoparticle graphite electrode, comprisingthe steps of: placing a solid graphite member into a tube filled with aprepared solution of L-ascorbic acid (AA) and Gold(III) chloride;placing the filled tube and solid graphite member combination into awater bath preheated to approximately 75° centigrade (C); retaining saidfilled tube and solid graphite member combination in said water bath forapproximately 15 minutes to form a gold nanoparticle graphite electrode(AuNP-GPE); removing said solid graphite member from said tube, saidsolid graphite member being transformed into said AuNP-GPE; washing saidAuNP-GPE in deionized water; drying said AuNP-GPE at approximately 60°C. for approximately 5 minutes prior to use; and cathodizing saidAuNP-GPE by placing said AuNP-GPE in a basic solution and applying −1.0volts to said AuNP-GPE for approximately 30 seconds.
 6. The method forforming a cathodized gold nanoparticle graphite electrode according toclaim 5, wherein said basic solution is a glucose analyte solution. 7.The method for forming a cathodized gold nanoparticle graphite electrodeaccording to claim 5, wherein said basic solution consists essentiallyof NaOH.
 8. The method for forming a cathodized gold nanoparticlegraphite electrode according to claim 5, wherein said basic solutionincludes NaOH and glucose.
 9. The method for forming a cathodized goldnanoparticle graphite electrode according to claim 5, wherein said basicsolution consists essentially of a 0.1 molar (M) NaOH including glucose.10. The method for forming a cathodized gold nanoparticle graphiteelectrode according to claim 5, further comprising the step of:preparing said AA and Gold (III) chloride solution in equal volumes,approximately 1.5 milliliters (ml) of each aqueous solutions ofapproximately 1.65 millimolar (mM) AA and 1.0 mM Gold(III) chloride,wherein said tube is approximately 3.0 ml in volume.
 11. The method forforming a cathodized gold nanoparticle graphite electrode according toclaim 5, wherein said solid graphite member is a mechanical pencil lead.12. The method for forming a cathodized gold nanoparticle graphiteelectrode according to claim 11, wherein said mechanical pencil lead isa hi-polymer graphite pencil HB grade lead.
 13. The method for forming acathodized gold nanoparticle graphite electrode according to claim 11,further comprising the step of: fixing an electrically conductive holderof said mechanical pencil lead vertically with 15 millimeters (mm) ofsaid mechanical pencil lead extruded outside said electricallyconductive holder and 10 mm of said mechanical pencil lead immersed in aglucose analyte solution as said basic solution, maintaining a geometricelectrode area of approximately 15.90 mm².
 14. The method for forming acathodized gold nanoparticle graphite electrode according to claim 5,wherein placement of said solid graphite member into the tube of AA andGold(III) chloride chemically deposits gold nanoparticles in the rangeof 20-85 nanometers (nm) on said solid graphite member.
 15. A method fordetecting glucose, comprising the steps of: placing a cathodized goldnanoparticle graphite electrode (AuNP-GPE) in a glucose analytesolution; and performing cyclic voltammetry on the glucose analytesolution using the cathodized AuNP-GPE to detect the glucose.
 16. Themethod for detecting glucose according to claim 15, further comprisingthe step of: using said cathodized AuNP-GPE to record cyclic voltammetryin the glucose analyte solution.
 17. The method for detecting glucoseaccording to claim 15, further comprising the step of: bubbling argongas in the glucose analyte solution to remove oxygen from the glucoseanalyte solution.
 18. The method for detecting glucose according toclaim 17, wherein the argon gas is bubbled in the glucose analytesolution for approximately 30 minutes to remove oxygen from the glucoseanalyte solution.